Having read the technical pdf that came with my Mega, I see the following :

Quote

The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin maysupply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheatand damage the board. The recommended range is 7 to 12 volts.

Can anyone please confirm that I understand this correctly :

The board can be powered by :

1. the USB port, taking the 5V USB power from the connected PC, or

2. use a LM7805 voltage regulator ( with 0.33uF cap between input & ground, and 0.1uF cap between output and ground ) on a heatsink to convert my 13,8V supply to a regulated 5V, and connect to the pins : 5V & Gnd, or

3. use a LM7808 voltage regulator ( same capacitors as mentioned above ) and connect to pins : Vin & Gnd, or

4. connect power to the 2.1mm power jack.

What I don't see, is what voltage the 2.1mm jack takes. Is it the 5V from the LM7805, or would it require the 8V from the LM7808 ?

That is the voltage they are referring to as external power.The difference between the jack and Vin is that there is a series diode in line with the jack for reverse polarity protection.If in doubt look at the schematic.Your other options are also valid.